1. Field of the Invention
The present invention relates to a method for making a lithographic printing plate whereby a negative-working photopolymer printing plate precursor is image-wise exposed, treated with water or an aqueous solution whereby at least a portion of the oxygen barrier layer is removed, mounted on a press, and processed on-press by applying ink and fountain solution to remove the unexposed areas from the support.
2. Description of the Related Art
In lithographic printing, a so-called printing master such as a printing plate is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a printed copy is obtained by applying ink to the image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e., ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e., water-accepting, ink-repelling) areas. In so-called “driographic” printing, the lithographic image consists of ink-accepting and ink-adhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the so-called computer-to-film (CtF) method, wherein various pre-press steps such as typeface selection, scanning, color separation, screening, trapping, layout, and imposition are accomplished digitally and each color selection is transferred to graphic arts film using an image-setter. After processing, the film can be used as a mask for the exposure of an imaging material called a plate precursor and after plate processing, a printing plate is obtained which can be used as a master. Since about 1995, the so-called ‘computer-to-plate’ (CtP) method has gained a lot of interest. This method, also called ‘direct-to-plate’, bypasses the creation of film because the digital document is transferred directly to a printing plate precursor by means of a so-called plate-setter. A printing plate precursor for CtP is often called a digital plate.
Digital plates can roughly be divided into three categories: (i) silver plates, which work according to the silver salt diffusion transfer mechanism; (ii) photopolymer plates which contain a photopolymerizable composition that hardens upon exposure to light; and (iii) thermal plates of which the imaging mechanism is triggered by heat or by light-to-heat conversion. Thermal plates are mainly sensitized for infrared lasers emitting at 830 nm or 1064 nm. Photopolymers can be sensitized for blue, green, or red light (i.e., wavelength range between 450 and 750 nm), for violet light (i.e., wavelength range between 350 and 450 nm) or for infrared light (i.e., wavelength range between 750 and 1500 nm). Laser sources have been increasingly used to expose a printing plate precursor which is sensitized to a corresponding laser wavelength. Typically, an Ar laser (488 nm) or a FD-YAG laser (532 nm) can be used for exposing a visible light sensitized photopolymer plate. The wide-scale availability of low cost blue or violet laser diodes, originally developed for data storage by means of DVD, has enabled the production of plate-setters operating at shorter wavelength. More specifically, semiconductor lasers emitting from 350 to 450 nm have been achieved using an InGaN material. An infrared laser diode emitting around 830 nm or a Nd-YAG laser emitting around 1060 nm can also be used.
Typically, a photopolymer plate is processed in an alkaline developer having a pH>10 and subsequently gummed to protect the plate from contamination, e.g., by oxidation, fingerprints, fats, oil or dust, or from damaging, e.g., by scratches during handling of the plate.
Photopolymer plates are also described in the literature wherein the wet processing and gumming steps are replaced by an on-press processing whereby the imaged precursor is mounted on a press and processed on-press by applying ink and fountain solution to remove the unexposed areas from the support. Methods for preparing such plates are disclosed in WO 93/05446, U.S. Pat. No. 6,027,857, U.S. Pat. No. 6,171,735, U.S. Pat. No. 6,420,089, U.S. Pat. No. 6,071,675, U.S. Pat. No. 6,245,481, U.S. Pat. No. 6,387,595, U.S. Pat. No. 6,482,571, U.S. Pat. No. 6,576,401, U.S. Pat. No. 6,548,222, WO 03/087939, U.S. 2003/0165777, and U.S. 2004/0013968. A compound for increasing the adhesion between the photopolymer coating and the support can also be added to the printing plate precursor for increasing the resistance of the exposed areas during the processing step and for improving the durability of the plate in the printing process as disclosed in EP 851 299, EP 1 091 251, U.S. 2004/0214105, EP 1 491 356, U.S. 2005/0039620, EP 1 495 866, EP 1 500 498, EP 1 520 694, and EP 1 557 262.
A problem associated with on-press processing of such photopolymer printing plates is the lack of daylight stability, i.e., the image is not stable before processing due to curing of the non-exposed areas by exposure to ambient light and, therefore, the exposed plate needs to be processed within a short time after the exposure. However, since on-press processing is not possible during a print job, the end-user must wait until the previous print job has been completed before the exposed plate can be mounted on the press and processed. As a result, the exposure of the plate for the next print job must be delayed until just before the completion of the previous print job, so as to avoid that the unprocessed plate is affected by the ambient light. Alternatively, the exposed plate must be kept under safe-light conditions, but this again reduces the ease of use and convenience that are normally associated with e.g., violet- and infrared-sensitive photopolymer plates.